Forcefulness shown by transformed Green Party finally (6)
I believe the answer is:
energy
'forcefulness' is the definition.
(energy is a kind of forcefulness)
'transformed green party finally' is the wordplay.
'transformed' is an anagram indicator.
'finally' says to take the final letters.
The last letter of 'party' is 'y'.
'green' anagrammed gives 'energ'.
'energ'+'y'='ENERGY'
'shown by' acts as a link.
(Other definitions for energy that I've seen before include "Capacity for action" , "Vitality, get-up-and-go" , "Power or vigorous activity" , "Pep" , "Vivacity" .)